1. Field of the Invention
The present invention relates to an organic synthesis, more particularly it relates to an improved process for producing highly purified benzenedicarboxylic acids isomers including terephthalic acid(TA), isophthalic acid(IA) and phthalic acid(PA), which are the most important monomers and semiproducts in a polymer chemistry for plastic, chemical fiber, film, varnishes and dyes.
2. Description of the Prior Art
Terephthalic acid is useful as a starting material for producing polyester films and fibers and is commonly produced by the so-called SD process wherein paraxylene is oxidized with molecular oxygen in the presence of catalysts containing heavy metals in acetic acid solvent. However, since the terephthalic acid produced from the SD process contains high content (1000-3000 ppm) of 4-carboxybenzaldehyde(hereinafter referred to as "4-CBA"), it is not a suitable starting material for producing polyester films and fibers.
Therefore, a method has been adopted in which terephthalic acid is reacted with methanol to form dimethyl terephthalate which is easily purified, and after purifying thereof, the dimethyl terephthalate is reacted with glycol to produce polyester. An alternative method which is more widely used for purifying terephthalic acid comprises the steps of dissolving the terephthalic acid in water at high temperature and pressure, treating the resulting solution with hydrogen over noble metal catalysts such as palladium in order to obtain highly purified terephthalic acid which contains less than 25 ppm of 4-CBA. However, the known processes have drawbacks; the former method produces methanol during the preparation process of polyester and the latter method requires two separate plants, each for oxidizing and for purifying, since the conditions of oxidation and purification, for example solvent, catalysts and operation conditions are different from each other.
Up to now, several methods have been proposed to avoid difficulties involved in these conventional processes.
Methods for producing terephthalic acid without reductive purification step wherein paraxylene is oxidized with molecular oxygen via consecutive 4 steps in the presence of cobalt-manganese-bromine catalyst in acetic acid had been proposed. [U.S. Pat. No.4772748, JP 62-270548A and JP 63-23982B] According to these methods, the paraxylene, in the first oxidation step, is oxidized at 180.degree.-230.degree. C. for 40-150 minutes with the conversion of more than 95%; in the second oxidation step, is oxidized at a temperature of lower than that in the first reaction vessel by 2.degree.-30.degree. C. for 20-90 minutes; in the third oxidation step, is oxidized at 235.degree.-290.degree. C. for 10-60 minutes; and the final fourth oxidation step, is oxidized at 260.degree. C. Since the TA produced by this method contains 0.027% of 4-CBA, it cannot be directly used for producing polyester fibers and films. Moreover, the above method has several disadvantages: a) since the high temperature (ca. 260.degree. C.) employed for oxidizing impurities in the third and fourth oxidation steps also causes oxidation of acetic acid solvent, the method is unfavorable in view of technology and economics; b) rather long reaction time in the first oxidation step decreases the efficiency of the process; and c) the content of 4-CBA contained in the TA as an impurity is still high (0.027%).
Accordingly, the above method is disadvantageous in that the efficiency of the process is relatively low and less purified TA is produced compared with the conventional method employing the reductive purification step.
Another method is proposed wherein paraxylene is oxidized with molecular oxygen in the presence of heavy metal compounds and a bromine compound in acetic acid medium with a conversion of higher than 90%, then the resulting mixture is crushed at 140.degree.-230.degree. C. in the molecular oxygen atmosphere to reduce the average diameter of terephthalic acid particles by more than 20% (the first purification step) followed by the second step wherein the slurry obtained in the first purification step is oxidized with molecular oxygen at a temperature of at least 10.degree. C. higher than that of the previous step and between 180 and 300.degree. C. [JP 60-233032A] The method gives pure TA which can be directly employed for the polymerization.
However, the method requires a separate equipment for crashing the terephthalic acid, for example an agitator of high speed rotation. Moreover, it is difficult to produce highly pure terephthalic acid containing 4-CBA of less than 0.0025%.
Further, another method has been proposed wherein the crude product slurry produced by liquid phase catalytic oxidation of paraxylene is treated with molecular oxygen in acetic acid medium in the presence of catalyst composed of compounds of cobalt, manganese, chromium, cerium, lead or their mixtures, the mount of said catalyst being 0.01-5.0% by weight of TA to be purified. [Germany Pat. No. 1270030] The method is disadvantageous in that since the treatment is effected at a temperature as high as 250.degree. C. for 1 hour, the acetic acid as well as impurities are oxidized.
Accordingly, there has been a need to provide an improved method for producing highly purified benzenedicarboxylic acid isomers without additional catalytic purification step.